The present invention relates to an electrochemical apparatus and method for measuring the amount of an oxidizable gas present in gaseous mixtures such as ambient air, and more particularly to an improved carbon monoxide detection cell and electrode structure and methods for the formation and operation thereof.
Electrochemical detection cells for measuring the amount of gases such as carbon monoxide and nitrogen oxides present in a gas phase mixture are known wherein the gas phase mixture and a liquid electrolyte are brought into reactive contact through a gas-permeable measuring electrode. The electrode and the electrolyte are selected such that the particular gas diffuses through the electrode and is oxidized at the electrode-electrolyte interface at a rate proportional to its concentration in the gas phase mixture. The oxidation reaction produces an electrical current whose level is directly proportional to the reaction rate and thus to the concentration of the gas in the gas phase mixture. Such cells may be of the galvanic type wherein the measuring electrode is used with a low resistance nonpolarizable reference electrode and the reaction causes a current flow spontaneously between the two electrodes without the application of an external voltage across them. Such cells may also be of the polarographic type, which is that of the present invention, wherein a predetermined external voltage is impressed across the measuring electrode and a reference electrode, which voltage, in the absence of the gas to be detected, polarizes the system to reduce the current between the electrodes substantially to zero. When the measuring electrode is exposed to the gas to be detected, a current flows which is a function of the concentration of the detected gas in the gas phase mixture. An auxiliary electrode is usually added to the measuring and reference electrodes to complete the electrical system.
One major problem with such electrochemical cells, which is particularly severe in detecting low concentrations of carbon monoxide, is that the measuring electrode becomes gradually irreversibly poisoned, and the reaction rate accordingly varies with such poisoning. As a result, the electric current sensed is no longer indicative of the concentration of the gas in the gas phase mixture without constant recalibration.
Heretofore, it has been the common belief in the art that the use of gold for the measuring electrode in carbon monoxide monitoring cells is infeasible because such gold electrodes will not oxidize carbon monoxide, in that gold is inert to carbon monoxide, as taught, for example in U.S. Pat. No. 3,776,832, Col. 12, lines 51-57. Further, it has been understood that the use of gold with a perchloric acid electrolyte will result in rapid (within seconds) and irreversible poisoning of the electrode as taught, for example, by Brummer, K.D.N., in "A Study of the Rate of Oxidation of Carbon on Three Dispersed Electrode Systems," Journal of Catalysis 9, 207-216 (1967). However, it has been discovered that with a gold-coated electrode made in accordance with the present invention, a highly sensitive carbon monoxide monitoring cell can be produced using substantially pure perchloric acid solution as the electrolyte, and that this electrode can be rapidly rejuvenated after each sampling, by suitably varying an imposed voltage to reverse poisoning, thus obviating frequent calibrations and resulting in a cell of an indefinitely long lifetime.